Distance measuring system



Aug. 26, i947.. R. c. Nam/HOUSE 2,423,232

DISTANCE'MEASURING SYSTEM Filed Feb. s, 1944 I4 /3 20 2l' 24 N' 22 svNcHRo vRANslFRE@ Moran urn-R DE T. N ANR METER L f cer EQUAL/21H5 /2 Nsrwa/m 52o Tf CAT/10D: 26 FIG. Rill/UBMRCIRIW R 27 d WM/977m 24 64 62 65 73 72 76V 59 .VHf/Q/ u u l K Il I nu nu HAMM ATTORNEY Patented ug. 26, 1947 DISTANCE MASURING SYSTEM Russell C. Newhouse, Millburn, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 5, 1944, Serial No. 521,173

' (ci. 25o-1.68)

3 Claims.

This invention relates to systems for measuring distances by theuse of radiant energy and particularly to such systems as are adapted for use as altimeters or terrain clearance indicators for aircraft.

More specifically, the invention relates to an improvement in the feedback circuit employed in the amplifier and frequency measuring portion of the receiver of the radiant energy distance measuring system disclosed and described in detail in U. S. Patent 2,206,903 issued July 9, 1940, to R. F. Lane and the present applicant jointly- The principal object of the invention is to improve the accuracy and reliability of the distance measurements particularly as the distances being measured become relatively quite small.w

In accordance with the above-mentioned patent and with this invention, distance is measured by radiating waves from one point to another and receiving the waves reflected back to the first point. The time interval between radiation and reception is a measure of the distance and is determined by cyclically varying the frequency of the transmitted waves at a known rate and over a known range. The frequency difference between the wave being transmitted and the received reflected wave is therefore a measure of the distance to the reectng surface. The measurements are obtained by beating together a portion of the transmitted wave and the echo wave and measuring the frequency of the resultant dierence frequency beat wave.

As a result, the accuracy of thel system is, of course, dependent upon the accuracy of the frequency measurement. This is'true not only with respect to the absolute measurement of the beat frequency wave but also with respect to the elimination from that measurement of any substantial eifect of currents of spurious frequencies. Such currents arise not only from such usual causes as circuit noise, vibration, power supply variations, harmonic production and the like but also from causes peculiar to the system itself. For example, amplitude modulation of the echo may be produced .by variation in the reflecting power of the terrain or by interference between component echoes from adjacent surfaces of different height or distance. Unless precautions are taken to eliminate such spurious frequencies quite false and misleading indications may be obtained.

The general principles of operation of the distance measuring systems with which the arrangements of the invention are intended to be 2 used are discussed in detail in the above-mentioned patent.

In the above-mentioned patent it is also taught that the majority of the above-described difculties can be substantially eliminated by giving the receiver-amplifier a frequency gain characteristic which rises with frequency and by providing means responsive to the particular beat frequency being measured toV limit the amplifier gain at frequencies higher than this particular beat frequency. This means of limiting the gain at higher frequencies comprises a feedback circuity which is described in detail in the above patent. The arrangements described in the patent have been found to be satisfactory over the greater portion of the range of altitudes or frequencies normally of interest in the navigation of aircraft.

However, at relatively low altitudes thear-` rangements of the patent have been found tobe less accurate and reliable than might be desirable, particularly for landing operations. This invention is therefore directed to a modification of the feedback circuit controlling the amplifier gain of systems of the type described in the above patent by which the accuracy and reliability of the system may be increased at relatively low altitudes.

With reference to Fig. 2 of the accompanying drawing and Fig. 5 of the above-mentioned patent, vacuum tube ed is used in a conventional form of resistance coupled amplifier circuit with constants so proportioned that the gain is substantially of constant value for all frequencies up to the highest frequency of interest. The vacuum tube is used in a feedback circuit which applies the amplified output of tube 64 tothe cathode circuit of the latter tube in the proper phase to reduce its output. The time constant of the coupling circuit including capacitor `591i and resistor 5% of the feedback circuit of-Fig. 2 or of network 23 of Fig. 5 of the patent, is chosen so that the maximum gain in the feedback circuit occurs at the highest frequency which is desired to have full gain through stage 24. This results in the Yfeedback circuit reducing the Yeffective gain at high frequencies more than at low frequencies, the reduction in effective gain falling on at the rate of 6 decibels per octave below-the point at which'the resistance is equal to the .reactance of the coupling circuit. Thus the'alternating current voltage across resistor 88 is controlled to provide a reduction in the effective gain which is substantially proportional to the aver-V age frequency of the signal being amplified in tube 64, and tube 85 is cut olf for high frequency signals but becomes operative to reduce the gain when the signal frequency is lower and gain at the higher frequencies is unnecessary and detrimental.

Referring again to Fig. 2 of the accompanying drawing and Fig. 5 of the above-mentioned joint patent, amplifier stages 24 and 26 constitute, respectively, a stage of the receiver-amplifier and a stage of the feedback circuit amplifier associated therewith. At high frequencies the reactances of the coupling circuit capacitors 8S and 504 of Fig. 2 or 86 and 8l of Fig. 5 of the patent are small as compared with the values of their respective associated resistors and the phase shift through the interstage coupling circuits is therefore negligible. The phase shift through the feedback circuit at high frequencies is therefore only that of the vacuum tube 85 which phase shift is 180 degrees. Vacuum tube circuit 52B of the present invention introduces no phase shift as will be explained hereinafter.

The operation of the circuit loop including vacuum tubes 6B and 85 is, at high frequencies, therefore, as follows. A positive signal on the grid of tube 64-causes an increase in the anode circuit current of the tube which flows through the cathode circuit resistor 88 and makes the cathode of tube Eil-more positive. At the same time the anode of tube 64 becomes less positive (or more negative). This causes the control grid circuit of tube 85 to become less positive (or more negative) at the same instant, since there is negligible phase shift and therefore negligible time delay in the circuit coupling the anode of tube fl and the control grid of tube 85. The anode circuit current of tube B5 is therefore reduced and the anode of tube 85 becomes more positive which raises the potential of the cathode of tube iid, either directly, as in the circuit of the patent, or through circuit 520 of the present invention, and tends to reduce -theanode circuit current thereof and the gain of stage Z4, as desired for normal operation of the complete circuit. As the frequency of the signal current is reduced, however, the' 'reactances of the coupling circuit capacitors 86 and 594 of Fig, 2 or 85 and 8l' of Fig. 5 of the patent increase and at the lower end ofthe frequency range become appreciable as compared with the values of their respective associated resistors.l At the lower extreme (i. e., zero frequency) the phase shift of each coupling circuit becomes'QO degrees so that the total phase shift of the feedback circuit becomes 360 degrees and the feedback, instead of being negative, is positive at'zero frequency. Fortunately, in the circuits ofthe patent and of the present invention7 the net loss raround the feedback loop is so high at zero frequency and at lower frequencies that singing or oscillation cannot take place. A limitation in using the original circuit (shown in Fig. 5`of the patent) arises from the fact that the cathode resistor 88 associated with tube 6d cannot be made large enough to be a satisfactory load at the lower end of the beat-note frequency range for tube 85 without being too large to permit satisfactory gain to be realized in tube 655. Additional gain at lower frequencies'cannot conveniently be obtained by the seemingly obvious expedient of adding a second stage of straightforward amplification in the feedback path since this would involve the introduction of yet another interstage coupling network which would add still more phase shift (reaching a maximum'value of feedback path with the result that the singing (or oscillation) frequency would be moved substantially upward in frequency from zero frequency and instability of the overall circuit,comprising amplifier stage 213 and the new 2 stage feedback circuit, would then be encountered at some low frequency point well within the range of frequencies employed in the operation of the system. The fundamental principles are, of course, well known in the art and are, for example, explained at length in an article by H. Nyquist, entitled Regeneration Theory, published in the Bell System Technical Journal, volume Xi, 1932, at pages 126 to 147.

To avoid the limitations described in detail above, this invention provides for the necessary additional gain at lower frequencies by interposing between the feedback stage 26, Fig. 2 of the accompanying drawings, and the cathode resistor SB of the amplifier stage 24, a special vacuum tube circuit 520 which increases the effective gain of the lfeedback stage 25 at lower frequencies by presenting at the output of stage 26 a suitably high impedance. rThe output of stage 25 is then stepped,r down (in impedance) by circuit 525i so that it can be connected across cathode resistor 33 without causing any substantial impedance irregularity at that point. The circuit 528 is of the well-known form designated in the art as a cathode-follower and introduces substantially Yno phase shift so that the original stability of the feedback circuit throughout the entire utilized range of frequencies is not disturbed. Capacitor illland resistor 5&6 which couple tubeA 592 to tube 35 can then present the desired higher impedance to tube 35 and at the same time provide the feedback Circuit with an attenuation-frequency characteristic which is substantially the same as that of network 2l of the main path of the amplifier. With respect to this latter function capacitor 5M and resistor 58B take the place of network 23 of Fig. 5 of the patent as intimated above.

The principles and nature of the invention will be more readily understood in connection with the following detailed description taken in conjunction with the accompanying drawing in-which:

Fig.V 1 shows in block-schematic diagram form an altimeter system foraircraft ofethel type in connection .with which the-principles of the vention may beV advantageously employed; and

Fig. 2 shows in electrical circuit schematic diagram form, the application of the principles of the invention to Jthe feedback circuit of an amplier, suitable for; use in the system illustrated in Fig. l..

In more detail,-in l the system comprises an ultra-short wave radio transmitter!! iequipped with a rotating condenser I'Z driven by a synchronous motor i3 for continuously varyingthe frequency of the transmitter. The output of the transmitter is connected throughv a coaxialV transmission line ltoahalf wave dipoleantenna I6 which is mountedra quarter wave length below the metal surface of Vthe wing ofthe airplane 29. upon which the system is carried, the .wing surface of the airplane acting asa reflector.

vA 'second similar half wave dipole. antenna Il',V

mounted in axiali alignment with the antenna i6 is connected through aY concentric transmission line i8 to a'detctor 2d. A s indicated in the diagram, the waves generated by the transmitter i l and radiated by the antenna I will be directlytransrnitted to the antenna il and also transmittedftheretoby reflection from they surface of therearth i9 or other objects, the-distance of which' it is desired to measure. The directly amazes transmitted and reflected signal components are applied to the detector il where they combine to produce a useful demodulation product which is a signal whose instantaneous frequency is equal to the instantaneous difference in the frequency of the two component waves. This difference frequency product is amplified and its frequency is measured and indicated by the frequency meter circuit 25, which is of the integrating type, and the meter 22. The average value of the frequency difference is a direct measure of the altitude, that is, the distance d between the measuring apparatus and the reecting surface i9.

The circuit for amplifying the frequency difference product includes the equalizing or attenuating network 2l and amplifier 2li which is provided with an inverse feedback circuit including vacuum tube circuits 20 and 520. The feedback amplifier 20 is provided with a control through an auxiliary frequency meter circuit 2. The operation and functions of these various ampliers and control circuits will be described in detail in connection with Fig. 2 which is a detailed electrical circuit schematic thereof.

The general principles of operation and the detailed description with appropriate illustrative diagrams and circuits are given for the other apparatus units of the system in my above-mentioned joint patent.

Amplifier and frequency measuring circuit Fig. 2 shows a detailed electrical schematic circuit of the amplifier and frequency measuring circuit of the radio receiver portion of the apparatus of Fig. l. This circuit is similar to that indicated in the block schematic of Fig. l and, in general, the same reference numerals are used for corresponding parts. There are shown, however, three amplirier stages l, 2li and 62 while in the block schematic of Fig. 1 the amplifier is indicated generally by the single block 2d.

As discussed above, and in my above-mentioned joint patent, the function of the amplifier circuit is to amplify the beat frequency output of the detector 20 which comprises waves, for example, of frequencies varying between 160 cycles and 40,000 cycles per second and to give an indication of the frequency thereof. The latter function is performed by the frequency meter or counter circuit E5 and the associated meter 22.

Vhen the equipment is operating at higher altitudes it has been found that there will be present, in addition to the difference frequencies representing the altitude, other lower frequencies primarily produced by amplitude modulation of the echo wave resulting from terrain irregularities and secondarily by amplitude modulation of the transmitter by noise and by other disturbing phenomena. Unless the effect of said frequencies is substantially eliminated, the frequency meter will not give a true indication of the height, or altitude, of the aircraft above the ground surface i0. It is, therefore, necessary to provide means for attenuating frequencies lower than the frequency representing the height or altitude. Thus, for example, if the aircraft is flying at 5,000 feet and the detector output signal is 40,000 cycles, it is desirable to attenuate frequencies below 40,000 cycles as much as it is conveniently possible. This is accomplished by giving the amplifier a frequency again characteristic which rises with frequency. It is possible to do this because the strength of the received echoes at the lower frequencies will normally be greater than at higher frequencies since the lower frequencies represent shorter paths (lower altitudes) for the ech'o transmission.

When the amplifier is given such a sloping frequency-gain characteristic, other difficulties are introduced in the operation of the system at lower altitudes where the frequency difference to be measured is in the lower frequency range. In such cases the increased amplification at highor frequencies may introduce distortion from undesired higher frequencies which' may result from the generation of harmonics of the desired signal, amplitude modulation of the echo by irregularities of the terrain as the aircraft fliestherealong and other miscellaneous causes. It is consequently necessary to provide means operative in the lower frequency range for limiting the degree of amplification of higher frequencies. This is accomplished by means of an automatically controlled feedback circuit as will be described presently in detail.

The output of the detector circuit containing the beat frequency wave which changes in frequency, in a typical example, from 1,60 cycles to 40,000 cycles as the altitude of the aircraft changes from 20 feet to 5,000 feet, is supplied to the amplifier through the connection 59. The main amplifier, as shown in Fig. 2, comprises three stages tl, 2d, and 62, employing pentode vacuum tube amplifiers 63, Ell and 05, respectively. The tubes are provided with conventional circuits for supplying screen and anode voltages and are coupled together through resistance-capacity circuits of the usual type. Two series-connected resistors 69.l and 69.2 shunted by a by-pass condenser 50 are provided in the cathode circuit of tube 03, This circuit provides negative feedback as will 3e described later. In addition the direct current voltage drop across resistor 69.1 is used for grid bias, being applied to the grid through the resistance-capacity filter 10, ll.

In the input to the first stage El there is provided an attenuating network 2l comprising the series condenser 66 and shunt resistor 0'! as shown in Fig. 2. The values of this condenser and resistor are so chosen that in combination with the cathode resistors 60.1 and 69.2 and the by-pass condenser 63 of the first amplifier tube 03, the maximum signal transmission occurs at some higher frequency, for example, at a frequency well above 20,000 cycles. F01 each decrease of one octave in frequency below 20,000 cycles, approximately 6 decibels additional attenuation of the signal is produced by the action of this network 2l in combination with the negative feedback action of the above-mentioned cathode resistor condens-er network.

By utilizing the negative feedback action of the cathode resistor 69.1, 69.2, and by-pass condenser 0B in combination with the network 2l, instead of the latter alone, for obtaining the desired frequency-gain characteristic of the amplifier, additional discrimination against noise is obtained. The reason for this is that in any multistage ampliiier, the chief source of circuit noise is the first amplifier stage. Since the negative feedback in the first amplifier tube decreases the gain of that tube at low frequencies, in which range most of the circuit noise lies, considerable discrimination against such disturbances is obtained.

The output of tube 63 is connected through a conventional resistance-capacity circuit to the input of the second amplifier tube @a which is in turn coupled through a similar circuit to thek input of the third amplifier tube 65. The circuit for the latter two tubes are so designed that their 7 respective stages provide substantially uniform amplification up to at least 40,000 cycles per second. The output of the amplifier tube 65 is connected to the frequency meter circuit 25.

The frequency meter circuit comprises a pentode tube i2, the grid circuit of which includes grid leak resistors 13, M and 75. The plate of tube 'l2 is connected through a resistor l5, which may be of about 20,000 ohms, to the constant voltage source 78. Between the plate and cathode of the tube l2, there is connected through the switch 19, a series circuit consisting of the condenser 80, and a bridge type rectifier 3i. A milliammeter 22 is connected across the remaining terminals of the bridge.

Starting at a time when no signal is applied to the grid of the tube l2, the internal resistance from plate to cathode of the tube is extremely low compared to the resistance of the resistor 16 so that the plate is practically at the same voltage as the cathode and the condenser 80 is discharged. If there is applied to the grid of tube 'l2 an alternating current voltage, the negative peak of which is sufficient to out off the plate circuit, the condenser Bil charges from the battery i8 through resistor l5. Provided the time the tube is blocked s long enough in comparison with the time constant of the condenser and resistance, the condenser 80 will fully charge to the voltage of the battery '58. The circuit is designed for this operation with respect to the current, the frequency of which is to be measured.

During the succeeding positive swing of the excitation applied to the grid of the tube '12, the condenser 80 is discharged to practically zero voltage. Since the action of the rectier 3l causes both the charging and discharging current of the condenser to flow through the meter 22 in the same direction, a positive deection occurs on rneter 22. A frequency of N cycles per second applied to the grid of the tube i2 causes a current equal approximately to ZNCE amperes to flow through the meter, where C is the capacity of the condenser in farads, and E is the plate supply Voltage. Thus it will be seen that the rectified current and consequently the meter deection will be proportional to the frequency of the exciting oscillations applied to the grid of tube l2 and independent of their amplitudes.

An auxiliary condenser 02 is provided Vwhich may be connected in circuit in place of the condenser 30 by means of the switch i9. This provides means for obtaining a diiferent scale reading on the meter 22. For example, if the condenser 30 were 100 mlcro-microfarads, this combination with a resistance of 20,000 ohms for the resistor l5, would provide a practically linear scale up to 40,000 cycles which corresponds to 5,000 feet. With a capacity for the condenser 82 of 500 micro-microfarads, the time constant of the circuit would be suitable for a maximum frequency of 8,000 cycles which cor 1esponds to 1,000 feet. Thus the use of switch le in connection with the two condenser circuits permitsthe use or the full deflection of the meter 22 for two different scale ranges of height.

The degenerative feedback circuitcomprising amplier stage 20 and vacuum tube circuit functions in the following manner. Y

Blocking condenser G Vserves to couple the grid of pentode 85 with the output, or anode, circuit ofampliiier stage The coupling circuit comprising series capacitor 50@ and shunt resistor 550 serves to couple the anode circuit of vacuum tube S with the control grid circuitrof vacuum tube 502. The coupling circuit 551i, 506 serves both to present a high impedance over the entire used range of frequencies so that the gain of tube is maintained substantially constant throughout that entire frequency range and also provides an attenuation in the feedback path which varies with frequency in the saine manner as that provided by network 2l of the main amplifier. With respect to this latter function, as mentioned above, the combination 50d, 505 takes the place of network 23 of Fig. 5 of my abovementioned joint patent. As a result of this arrangement the feedback circuit can be made to effectively compensate for the sloping characteristic of the main path amplifier so that the combination of main and feedback amplifier paths can provide an overall amplifier system, the amplification of which is substantially constant with frequency. The point at which the feedback amplifier will become effective to produce such a result will depend upon the amount of gain in the feedback path. Since, as discussed above, it is desirable that the amplifier .ave less gain for frequencies below the frequency representing the height being measured, it is likewise desirable that the feedback amplier be only effective to flatten out the overall gain characteristic of the main amplifier stage 2Q for frequencies above the frequency representing the height being measured. In order to accomplish this end, the plification of amplifier 23 and circuit 520 is controlled accordance with frequency.

For this purpose there is provided an auxiliary frequency metering circuit 27 which in general design and principle of operation is similar to the main frequency meter circuit 25. This circuit comprises a, pentode vacuum tube 50 which has its grid connected in parallel with the grid of the tube l2 through the blocking condenser Si. The grid is connected to the cathode through a grid leak resistor S2 and a negative biasing battery 03. The plate circuit of the tube 00 corriprises a resistor .Fifi connected in series with' the plate battery and in parallel therewith from the plate to the cathode a series circuit comprisv ing a condenser 96, two half wave rectiers 97 and 58 and the resistor 09.

This circuit operates in the saine way as the frequency meter circuit 25 and the rectiiiers 97 and 98 are so connected that the voltage drop across the resistor 99 is negative with respect to ground. The negative voltage developed across the resistor 99 plus an additional negative voltage supplied by battery E05 is applied to the control grid of the tube t5 through' the resistance-capacity filter lli. Since the voltage drop across resistor 99 is proportional to the frequency impressed on the grid of the tube 9i), the control bias supplied to the control grid of the tube 85 will also be proportional to frequency. As a result the amount of degenerative feedback provided by amplier 26 and circuit 520 is made to decrease with frequency.

The overall effect of the ciruit arrangement just described is in general similar to that illustrated by the characteristic curves shown in Fig. 6 of my above-mentioned joint patent. However, the addition of the circuit 525 substantially increases the Veifectiveness and reliability of the feedback arrangement at lower heights without any sacrice in the'operational characteristics at the higher end of the frequency range. By way of example, systems of the present invention provide substantially more accurate and reliable altitude indications below 100 feet than systems of my above-mentioned joint patent. The value of the system as an aid in landing aircraft is substantially increased thereby.

In circuit 520 resistors 522 and Sl serve to isolate their respective circuits from each other while permitting them to obtain suitable operating voltage from the common potential source 522 through the filtering circuit which comprises series resistors 524 and shunt capacitors 526. Capacitors 5ii8 and Elli by-pass higher frequencies which might otherwise cause interference in the battery potential supply circuit of source 522. The remaining portions of the altimeter system of Fig. 1 may be precisely as described for the system employed for purposes of illustration in my above-mentioned joint patent.

Numerous other arrangements embodying the principles of the present invention can be readily devised. by those skilled in the art. The invention is defined in the following claims.

What is claimed is:

l. In combination with a distance determining system including means for receiving a plurality of frequencies and for obtaining a wave having a frequency representing the distance being determined, a rst means connected thereto for attenuating frequencies below that of the wave and amplifying said wave, means controlled by the first means for obtaining a first Voltage having an amplitude representing the first-mentioned frequency, a circuit including a second amplifying means coupled in feedback relation with said first means, said second amplifying means including means in the output thereof for increasing the effectiveness of said second amplifying means at low frequencies without increasing the phase shift thereof, and means for introducing the said rst voltage into the gain control circuit of said second amplifying means to control the feedback action of said second amplifying means at frequencies above that of said distance representing wave.

2. In a system for determining distance between an oibject and a reflecting surface, a first means at said object for transmitting to said surface a radiant energy wave having a continuously varying frequency, a second means at said object for simultaneously receiving said wave after reflection from the surface and a wave directly propagated from the rst means to the second means, means for obtaining from said waves a current having a frequency representing the distance, means for amplifying said current and regulating means controlled in accordance with the frequency of said current for regulating the response of the amplifying means to other frequencies, said regulating means including a feedback path comprising an input amplifier stage and an output circuit, the output circuit including means to preserve substantially uniform impedance relations between the amplifier stage and the output of the feedback path over the entire frequency range of interest and more especially at the lower frequencies of said range whereby the effective gain of said feedback path at the lower frequencies is increased without increasing the phase-shift thereof and the effectiveness of the system at short distances is substantially increased.

3. In a system for measuring the altitude of an aircraft above the surface of the earth, a radio transmitter on said object for transmitting waves to the earth, means for cyclically Varying the frequency oi the waves transmitted thereby, means on said object for receiving said waves after reection from the earth and simultaneously receiving waves directly from said transmitter, means for combining the reflected waves and the direct waves to produce a beat frequency wave, means for amplifying the beat frequency wave, said last-stated means having a response directly proportional to frequency for at least a portion of the range of frequencies to -be amplified, means for obtaining an indication proportional to the frequency of the wave being amplified, a degenerative feedback path operatively coupled to said amplifier means and having a transmission characteristic directly proportional to frequency, means responsive to the frequency of the wave being amplified for controlling the transmission cf said feedback path in inverse proportion to the frequency of said wave throughout at least a portion of the range of frequencies to be amplified, said feedback path including an input amplifier stage and an output circuit having substantially no phase shift, said output circuit including a cathode follower vacuum tube circuit and substantially eliminating impedance irregularities between the feedback circuit and said first and seco-nd stated amplifying means over a broad range of frequencies whereby adequate and substantially uniform gain is provided in said feedback path without increasing the phase shift thereof and more accurate and reliable indications are obtained especially at lower frequencies.

RUSSELL C. NEWI-IOUSE.

REFERENCES YCITED UNITED STATES PATENTS Name Date Lane et al. July 9, 1940 Number 

